1. Field of the Invention
This invention relates to a low thermal expansion resin material comprising a polyimide having a special chemical structure and being oriented and a composite shaped article using the same.
2. Description of the Prior Art
Organic polymers have by far larger coefficients of thermal expansion (coefficient of linear expansion), for example 4.times.10.sup.-5 K.sup.-1 or larger in most cases, even at a temperature range below the glass transition temperature compared with metals and inorganic materials. Problems caused by such large coefficients of linear expansion of organic materials are remarkably numerous. It is not too much to say that all the reasons for not progressing as desired in applications and development of organic polymers are based on this characteristic. For example, in a flexible printed circuit (FPC) comprising a film and a conductor, there is desired a film obtained by coating or hot pressing a flexible film material on a metal foil. But since it is necessary to cure and dry at a high temperature after the coating or to hot press, there arises a problem of curling of the film material due to thermal stress caused by difference in coefficients of thermal expansion after being cooled to room temperature. Usually, in order to avoid such a problem, the film and the conductor are laminated by using an adhesive which can be cured at low temperatures. But in the case of FPC which is required to have heat resistance, since the adhesive curable at low temperatures is generally poor in heat resistance, a goodheat resistance for the laminate inherently cannot be exhibited even if a heat resistant film such as a polyimide film is used as substrate. On the other hand, in the case of coating, when an organic polymer is coated on a metal plate or inorganic material having a very small coefficient of thermal expansion compared with the organic polymer, there take place deformation, cracks of the film, peeling off of the film, breaking of substrate due to thermal stress caused by differences in coefficients of thermal expansion. For example, when a coating film is formed on a silicon wafer as a protective film for LSI (large-scale integrated circuit) or IC (integrated circuit), the wafer is warped, which results in making photolithography for patterning impossible, making resolving power extremely worse, or in the case of large thermal stress, peeling a passivation film, or sometimes causing cleavage and breaking of silicon wafer per se.
As mentioned above, there are a large number of problems caused by the large coefficient of linear expansion of organic polymers and for a longtime has been a desire for organic polymers having a low thermal expansion coefficient.
Under such circumstances, the present inventors have studied in detail the relationship between chemical structure and the thermal expansion coefficient by using many heat resistant resin materials, particularly by using various polyimides. There have been provided various kinds of polyimides, but the number of practically synthesized or polyimides in use are very small. Heretofore, practically synthesized, reported or marketed polyimides are those obtained by using as raw materials an aromatic diamine such as diaminodiphenyl ether, diaminodiphenylmethane, para-phenylenediamine or diaminodiphenyl sulfide, and a tetracarboxylic dianhydride such as pyromellitic dianhydride, benzophenonetetracarboxylic dianhydride, tetracarboxydiphenyl ether dianhydride, or butanetetracarboxylic dianhydride. But these polyimides have a remarkably large coefficient of linear expansion of 4 to 6.times.10.sup.-5 K.sup.-1.
But the present inventors have found that polyimides having an extremely small linear expansion coefficient and remarkably excellent tensile strength compared with those mentioned above can be obtained from a special aromatic diamine and a tetracarboxylic dianhydride mentioned below. This invention is based on such a finding.